This invention relates to a panel structure made of composite materials.
In one form of construction, a portion of an aircraft fuselage or other structure is formed of flat or curved structural panels. The structural panels are relatively thin compared to their lateral extent in two dimensions. For maximum structural efficiency, the structural panels are used without a separate strengthening framework. This type of construction, sometimes termed a monocoque structure, is to be distinguished from a structure wherein a separate internal framework provides most of the structural strength, and panels are affixed over the framework.
In this type of construction, the structural panels themselves must have mechanical properties sufficient to carry imposed structural and aerodynamic loadings. For thin structural panels, one of the most demanding and difficult-to-achieve properties is sufficient structural stiffness. That is, the structural panels must be resistant to buckling under load.
In conventional practice, such structural panels are made of a honeycomb sandwich panel structure. The honeycomb sandwich panel structure is formed of two facing, but spaced-apart, face sheets, separated by a core of an expanded metal or other type of honeycomb structure. The face sheets, which may be made of a composite material, provide load carrying capability, and the core spaces the face sheets apart by the amount required to achieve the required panel stability.
The present inventor has recognized that such honeycomb structures, while operable, have some disadvantages. Sandwich panels tend to be rather fragile. Damage to the structural panel usually is initiated from the exterior side of the panel, and the interior face sheet cannot aid in resisting such external damage. Further, the presence of the face sheet on the interior surface inhibits the inspection of the honeycomb interior and the face sheet/honeycomb bond lines of the structural panel for damage, and repair of that damage when it occurs.
Face sheet/honeycomb/face sheet structural panels are often not straight and have features therein. The fibers in the face sheets are therefore not straight, and cannot bear their full design loadings. The usual response to this situation is to make the face sheets thicker and therefore heavier.
Yet another problem is that the honeycomb material is sometimes selected to be a material that is light in weight but absorbs moisture. The increase in weight due to absorbed moisture may change the weight distribution and balance of elements such as flight control surfaces.
Available face sheet/honeycomb/face sheet structural panels are operable, but have significant drawbacks in some applications. There is therefore a need for an improved type of structural panel. The present invention provides such a structural panel, and further provides related advantages.